In many applications it is necessary to use metal components having a metal surface coating giving special properties such as resistance to corrosion, surface hardness, resistance to abrasion or the like.
One example of such treatment is chromium-plating, which is carried out on some moving mechanical components such as actuator shafts or rods, runners or slides or the like, since these components need high surface mechanical strength or resistance to corrosion in the operating environment.
In methods of electroplating used for this purpose, a layer of electroplated material is deposited on the metal surface to be treated, the layer being supplied in the form of positive ions in an electrolytic bath in which the metal component to be coated forms the cathode, a voltage being applied so as to cause the required current to flow.
In order to carry out this operation continuously, more particularly on rectilinear bars and the like, there are some known cells in which each bar is inserted through a mouthpiece fitted with sealing means and is immersed in the electroplating bath and travels inside one or more tubular anodes having perforated surfaces and likewise immersed in the bath.
These cells, however, present some problems in that when the bar is in direct contact with the bath, a stationary layer of bath forms during treatment, in which the concentration of the metal ion to be deposited is lower than the optimum value and thus slows down the deposition process. Also, interfering reactions occur in the electrolytic bath and form gas, more particularly hydrogen, at the cathode surface.
In order to remove the gas, the anode is given a perforated surface, but hydrogen bubbles collect on the cathode surface and thus separate it from the bath and impede chemical deposition in these areas, resulting in irregularities and defects in the deposited layer, e.g. porosity or reduced compactness, thus reducing the chemical and mechanical strength imparted by the treatment.
This process also limits the maximum density of the current which can be applied to the cell, and consequently limits the speed of electroplating since, above a certain limit, an increase in current results in a substantial increase in side-reactions without increasing the deposition of metal on the cathode, owing to the limited exchange of intermediate chemical species inside the cathode film.
Another requirement is that the surface of the bar to be processed must be activated in order to increase the efficiency of electroplating. Activation is advantageously brought about by chemical means, with a controlled attack in the electroplating bath, but there is then the problem of providing a zone in the electroplating cell where the electrochemical potentials enable chemical action to occur to the desired extent.
In known cells, there is also the problem of ensuring a good seal around the bar moving across the cells, to avoid losses or escapes of the bath to the exterior, and also of adapting the seals to differences in dimensions between successively introduced bars, so that for example bars of different dimensions can be treated in succession without interruptions through replacing the sealing means between groups of bars of different dimensions.